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-   -   Why aircrafts are producing AC instead of DC ? (https://www.pprune.org/tech-log/604137-why-aircrafts-producing-ac-instead-dc.html)

topla abicim 13th Jan 2018 06:55

Why aircrafts are producing AC instead of DC ?
 
Hi guy I search all the forums but there is no clear explanation.

wiggy 13th Jan 2018 07:14

FYI there are/were aircraft around that “produce” DC .....and they then often have to have inverters installed to get the AC power needed for any components needing such a supply.

Leaving aside the school answer about AC and transmission losses (which may or may not be significant with airframes) I’m guessing AC has the advantage of being able to be transformed/rectified to other voltages and/or rectified to DC as and where required around the airframe...it might be harder/less convenient to do the process the other way.

Awaits incoming....

riff_raff 13th Jan 2018 07:33

There are two types of AC power generators. Variable frequency and constant frequency. Constant frequency generators (or IDG's) use a variable speed transmission to maintain a constant speed at the generator as the engine drive speed varies.

Basil 13th Jan 2018 09:30

Not related to the OP but high voltage DC is now being used for long range power transmission. Mr Tesla would be disappointed ;)

andmiz 13th Jan 2018 09:50

AC wiring is much thinner and lighter.

Goldenrivett 13th Jan 2018 10:57

Wire needs the same dimensions to carry the same current - whether its AC or DC. The heating effect is I squared * R - doesn't make any difference to the direction of electrons.

wiedehopf 13th Jan 2018 11:00

DC motors and generators need brushes. Those wear out leading to increased maintenance and make sparks for a bonus fire hazard. Not something you want on an airplane.

You would rather have:
https://en.wikipedia.org/wiki/Induction_motor
https://en.wikipedia.org/wiki/Induction_generator

You could rectify the AC generator output but you would still need brushed motors and more importantly:

Low-voltage circuit breakers are also made for direct-current (DC) applications, such as DC for subway lines. Direct current requires special breakers because the arc is continuous—unlike an AC arc, which tends to go out on each half cycle. A direct current circuit breaker has blow-out coils that generate a magnetic field that rapidly stretches the arc.
(https://en.wikipedia.org/wiki/Circui...rcuit_breakers)

Switching 115V DC (equivalent cabling requirements to 115V AC) is not trivial at all, and going lower voltage would mean much more weight for thicker copper wires carrying all the extra current. Also what i quoted above is not just for breakers but also for normal switches and relais.

Not sure about the weight of DC motors compared to AC 400Hz motors but i guess the 400Hz wins again and the brushes i already mentioned.


I'm pretty sure the extra problems with switching DC and of course not being able to put it through a transformer are both more or less instant killers without even looking at motors/generators.


Today you can transform DC with switch mode transformers so it's not so much an impossibility compared to 50 years ago but then comes reliability again.

and @andmiz AC wiring at the the same voltage and current is not thinner and lighter.
DC used on cars and trucks though is much lower voltage due to the switching problem and batteries being low voltage so that makes the wiring much heavier.

andmiz 13th Jan 2018 11:14


Originally Posted by Goldenrivett (Post 10018518)
???

Wire needs the same dimensions to carry the same current - whether its AC or DC. The heating effect is I squared * R - doesn't make any difference to the direction of electrons.

At low voltages, large DC currents must be carried by larger, heavier wires. The associated components are also heavier, so weight becomes a major disadvantage. Higher voltage AC is more practical with its light weight components. High voltage AC power is preferred over DC for large aircraft mainly due to weight and space savings and low current transmission. AC generators are lighter and smaller than DC generators of a similar output.

wiedehopf 13th Jan 2018 11:33

lower voltage means heavier cables that's correct. now that's not what you wrote, you wrote about AC vs DC not low vs high voltage.

of course it's kinda correct because DC used for these applications is usually lower voltage.

mnttech 13th Jan 2018 12:39

What seems to be missing out of this discussion is Power. Every electrical item the designers install use it.
As a review: Power (P) = Current (I) times Voltage (E) or P=IE.
That can then also be reworked to P/E = I, which is where the wire size comes into play.
For example,
If a landing light consumes 250 Watts, a typical value:
@ 14 V (AC or DC, it does not matter) that is 17 Amps (round values)
@ 28 V that is 9 Amps
@ 115 V that is 2 Amps
Now, let us look at the wire size to carry that. From the circuit breaker in the cockpit to the wing root on Boeing 707 (think back when this was first being done, and probably more like a DC 4) that wire run would be about 80 feet.
To carry 14 Volts @ 9 Amps and only have 1 volt drop (allowed for intermittent loads like landing lights) an 8 AWG wire is required, which weights in around 5 lbs and is 0.17 in diameter.
28 V at 9 Amps uses a 12 AWG wire and 2 volt drop which weights in around 2 lbs, and is .09 in diameter
115 V at 2 Amps is smaller than 24 AWG and an 8 volt drop. Back then about the smallest that was used was 20 AWG which weights in around 1/2 lbs at 0.04 in diameter.

As noted, designs have changed in time and we can now generate higher voltage DC systems. Back in the 707 day the only way was AC systems. In addition, the AC system was 3 phase, which helps efficiency in things like motors and power conversion systems. DC switching systems now change all of that.

Chu Chu 13th Jan 2018 13:28

Until the 1960s or so, most cars had "generators," which produced DC. They had heavy permanent magnets, and brushes and commutators that wore out. Around that decade, cars switched to "alternators," which generate AC using field coils, with no permanent magnets or brushes. They're smaller, lighter, more reliable, and generally have higher output. In a car, the AC output is rectified to DC by diodes contained in the alternator itself. In many aircraft, the power is distributed as AC for the reasons others have pointed out.

aterpster 13th Jan 2018 13:30

Why we use 115v 400Hz power supply in aircraft? | GoHz.com

Chu Chu 13th Jan 2018 13:37

Just noticed no one said this directly: AC can simply and efficiently be converted to a higher or lower voltage through a simple transformer. So in an aircraft, power can be distributed at a relatively high voltage, permitting lighter wiring, and then be converted to a lower voltage (and then to DC) where needed. This is the same reason utilities transmit power over long distances at tens of thousands of volts, covert that to thousands of volts at substations, and then to 120 or 240V at transformers on street corners.

As others have pointed out, its now much easier to convert DC between voltages, but it's still not as simple or efficient as with AC.

megan 14th Jan 2018 04:38

And in AC you have the power factor to consider. AC/DC not an apple to apple comparison.

https://en.wikipedia.org/wiki/Power_factor

FE Hoppy 14th Jan 2018 12:18

AC= brushless generation( less rf noise, no sparks, longer life, lighter weight)
115v by convention
400hz, well latest trend is (back to )variable frequency but high because it saves weight on rectification.

wiedehopf 14th Jan 2018 14:51

high frequency means smaller motors and smaller generators as far as i know.

Capt Pit Bull 19th Jan 2018 08:04

You've got 3 options for electrical power distribution:

DC
Constant Frequency AC
Frequency Wild AC

Lots of relevant individual points made above already regarding the different options. There are advantages and disadvantages to each. There are essentially 3 questions to ask when looking at power system design. They all relate to your consumers (i.e. the equipment that needs the power).

1. Do they need a lot of power or just a little bit?
2. Are they resistive in nature (i.e. do they tend to draw current in phase with the voltage) or are they reactive (i.e. draw current out of phase).
3. How much stored power is needed in the event of the loss of power generation.

If you don't need a lot of power then the simplicity of DC wins, and your emergency supply is a battery.

If you need a lot of power then the various advantages of AC win.

If you have a reasonable amount of reactive load to deal with, then your system is best optimised by balancing the load to improve the power factor (as mentioned earlier) but this requires a constant frequency system. Change the freq and the balancing goes out of whack. Hence we have the overhead of things like CSDUs / IDGS. We're hauling extra weight, but it is worth it in this case.

However if the majority of your load is resistive then frequency wild can be a good solution. e.g. turbo props. Might need a lot of power for ice protection due to medium level ops.

Where there is a small need for constant frequency AC the most practical way to provide it might be via DC driven inverters

Aircraft electrical system design mixes and matches these three types of power system according to the requirements of the power consumers.

Alpine Flyer 19th Jan 2018 08:37

Some of the power requirements are probably legacy, too. I remember the Dash 7 and 8 having a mainly DC system with variable AC used for de-icing and some pumps only, but they still needed transformers and TRUs for some sytems.

Another relevant factor is that Constant Speed Drive generators are more complex and probably heavier than variable AC and DC generators.

FE Hoppy 19th Jan 2018 10:58

Power factor is a red herring in this discussion.

Weight and Reliability are where it's at.
High voltage means lighter cables
High frequency means less metal in the motors and TRUs

megan 19th Jan 2018 14:15


Power factor is a red herring in this discussion
These would suggest otherwise.

https://www.casa.gov.au/file/152026/...token=XwhMsWDd

http://www.martekpower.com/pdfs/en/72.pdf

Analysis of a C-130 electrical system. PF gets quite a mention.

http://www.collectionscanada.gc.ca/o...01/MQ44836.pdf


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